A Pb-acid battery stores and releases energy by electrochemical reaction(s) at the surfaces of its electrodes. Each cell in the fully charged state contains electrodes of elemental lead (Pb) and lead (IV) dioxide (PbO2) in an electrolyte of dilute sulfuric acid (H2SO4). In the discharged state both electrodes turn into lead (II) sulfate (PbSO4) and the electrolyte loses its dissolved sulfuric acid and becomes primarily water. In the pasted-plate construction each plate consists of a lead grid initially filled with a paste comprising a mixture of leady oxide (Pb and PbO) and dilute sulfuric acid. This construction allows the acid in the paste to react with the leady oxide inside the plate during cell formation (first charge and discharge cycle during which linkages occur between neighbouring particles), increasing the electrical conductivity and active surface area and thus the battery capacity. The paste may also contain carbon black, blanc fixe (fine barium sulfate), and lignosulfonate.
Vehicle hybridisation driven by increasing worldwide demand for lower automotive emissions and/or increased fuel economy places increased demand on vehicle batteries, which are most commonly Pb-acid batteries. For example the European Union has set a long-term emissions target of not more than 95 g carbon dioxide/km to be reached by 2020 for new vehicles.
Many new internal combustion engine (petrol, diesel, or gas) powered vehicles also have idle elimination functionality—the engine is arranged to switch off when the vehicle is stationary or travelling at low speed. Such vehicles are referred to as stop-start vehicles or micro-hybrid vehicles. Each engine restart draws energy from the battery and if this occurs more quickly than energy can be replaced by recharging, during only relatively short engine on periods in commuter traffic for example, the battery charge (or state of charge) will not be maintained. Current is also drawn from the battery during periods in which the vehicle engine is off to maintain other functionalities in the vehicle such as air-conditioning, radio etc (referred to as “hotel loads”). Battery charge may fall sufficiently that the vehicle battery management system will then override the idle elimination functionality to prevent any further engine stop-starts until the battery's state of charge has recovered. Thus to maintain battery charge in even for example heavy commuter traffic a battery for such a stop-start or micro-hybrid vehicle should have a high dynamic charge acceptance (DCA) rate, which refers to the rate at which a battery will accept charge.
Vehicles with a higher level of hybridisation including vehicles comprising both an internal combustion engine and an electric motor typically comprise regenerative braking, in which braking force is applied by a generator the electric energy from which is stored in the vehicle battery. The vehicle battery is charged only by current from regenerative braking during time periods in which the internal combustion engine which also drives a generator (which here includes alternator) is not operating. Under regenerative braking relatively high charging currents are supplied to the vehicle battery for short time periods and thus batteries for hybrid vehicles with regenerative braking should also have high DCA. Full electric vehicles also comprise regenerative braking.
The charging system of a hybrid vehicle is arranged to use the engine-driven generator to maintain the charge state of the vehicle battery at less than full charge such as for example at about 80% charge, so that there is generally capacity available to accept additional charging energy from regenerative braking. However the battery DCA then typically declines over time with increasing number of discharge and charge (to less than full charge) cycles, with AGM batteries typically operating at around 0.1 to 0.3 A/Ah (or 0.1 to 0.3 C) within a few thousand cycles. This loss in charge acceptance reduces the fuel saving capability of the vehicle; automakers ideally want a battery that can accept up to 2 A/ah, or even 3 A/Ah over a 5 to 10 second period to maximise the fuel saving potential of the start/stop and regenerative braking functions. However, any improvement above the 0.1 to 0.3 A/Ah is a valuable improvement. Typically the charging system of a hybrid vehicle is arranged to allow the battery to discharge and then (using the engine-driven generator to) charge the battery. Generally, the cars Battery Management System will periodically fully charge the battery (or “recondition” the battery) to restore the battery DCA, such as every three months. An ideal Pb-acid battery, particularly for a hybrid vehicle, would maintain DCA without requiring periodic full charging, or at least would maintain a higher rate of DCA between reconditioning cycles.
In a Pb-acid battery DCA is primarily determined by the charging reaction at the negative electrode.
A battery should also meet other requirements, such as have high volumetric energy density. Volumetric energy density (VED) refers to the energy supplied per unit volume of electrode. A closed Pb-acid battery system should also have low water consumption. And an automotive battery in particular should be able to deliver high current for engine starting, at low temperature. A cold cranking amps (CCA) test tests the ability of a battery to do so.
U.S. Pat. No. 7,569,514 describes utilising activated carbon as an electrode in an absorbed glass mat battery to overcome sulphation to thereby increase the dynamic charge acceptance of the battery.
U.S. Pat. No. 4,429,442 describes a lead-acid battery plate comprising a metal grid and active mass and a layer of carbon fibrous material on the side of the active mass to enhance mechanical integrity of the active mass.
U.S. Pat. No. 4,342,343 describes a negative lead-acid storage battery plate with interconnected carbon fibres over the face of a pasted plate. During manufacture formability is enhanced by securing the fibres to a paper carrier and then pressing the same to the plate.
U.S. Pat. No. 6,617,071 describes an electrode having a conductive polymeric matrix formed over the surface of a grid plate where the conductive polymeric matrix comprises superfine or nanoscale particles of active material.
Our international patent application publication WO2011/078707 discloses a lead-acid battery comprising as a current collector a conductive fibrous material of filaments with low interfibre spacing and conducting chains of Pb-based particles attached to the fibres, which provides improved battery performance particularly DCA.